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Nearby Tau Ceti may host two planets suited to life

By Jacob Aron

A star that is a mainstay of science fiction has surged ahead in the race to find life-friendly alien planets. If the discovery of two planets orbiting Tau Ceti is confirmed, they will be the closest potentially habitable exoplanets yet discovered.

Tau Ceti’s proximity and similarity to our sun have captured the imagination of writers and made it a promising candidate for life outside our solar system. The star was the target of one of the earliest searches for extraterrestrial life, in 1960, but astronomers had been struggling to see any hints of orbiting planets.

Now Hugh Jones at the University of Hertfordshire, UK, and colleagues have reanalysed observations of Tau Ceti using a new technique that can filter out noise to reveal previously hidden planets. The team discovered five potential planets ranging from two to nearly seven times the mass of Earth, with orbits ranging from 14 to 640 Earth days long.

The highlight of this alien solar system is Tau Ceti e, which has a mass of over four Earths and a year just under half as long as ours. It orbits in the star’s habitable zone, the region where liquid water is thought to exist. “It is in the right place to be interesting,” says Jones.

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Abel Méndez at the University of Puerto Rico at Arecibo has independently analysed the data and says that the fifth planet, Tau Ceti f, may also be in the habitable zone. Jones is cautious, however. “It’s not as strong a case, it’s only just in the habitable zone,” he says.

Wobbly clue

Tau Ceti’s five planets were discovered by looking for the small gravitational wobbles they induce in cause the parent star. These wobbles are minute and often obscured by other noise in the data, but Jones’s team created a new statistical technique capable of teasing them out. Their work will be published in Astronomy and Astrophysics.

Tau Ceti, only 12 light years away, was chosen as a test case because no planets had been found there previously despite the volume of observational data available. Now the technique can be applied to other stars as well. “If these signals hold up as planets, we will likely see their methods used on many more stars,” says Philip Muirhead of the California Institute of Technology in Pasadena, who was not involved in the latest work.

The five Tau Ceti planets join a list of over 800 previously discovered candidate exoplanets, including Alpha Centauri B b, which was found earlier this year and is the closest known exoplanet at just 4 light years away. It is too near to its star to be habitable, but could indicate the presence of other, more life-friendly worlds that we have yet to discover. Unlike Tau Ceti and our sun, its host star, Alpha Centauri B, is part of a binary system along with another star, Alpha Centauri A.

Asteroid glut

The Tau Ceti system has its own quirks though, including a large asteroid belt, with 10 times more asteroids and comets than our own solar system. That could harm the chances of life existing, says Jones. “More material around seems like a bad thing in terms of more impacts.” Asteroids can also bring water to a planet, however, which could be beneficial.

Xavier Dumusque of the Geneva Observatory in Switzerland, who led the discovery of Alpha Centauri B b, says Tau Ceti e seems promising, but adds that the new technique used by Jones’s team has to be independently verified. “If the planet is really there, it would be the best candidate so far to harbour life,” he says.